This invention relates to a method and apparatus for coating a moving web with a plurality of coatings and more particularly to a method and apparatus for coating a moving web with a multilayer liquid composition which is subsequently set or gelled on the web.
Presently, there is available coating apparatus for applying a plurality of superimposed layers to a moving web. Typical apparatus utilizes a plurality of inclined slide surfaces separated by exit slots through each of which is metered a coating solution onto an adjacent inclined surface. The coating solutions flow by gravity over the inclined surfaces and those metered through upstream slots flow over coating solutions metered through downstream slots and form a multilayered stream formed of the individual coating solutions that cascades over the downstream inclined surfaces. As the coating solutions flow under gravity over the inclined surfaces, each layer becomes smooth and is of uniform thickness. At the last inclined surface or slide, the multilayered stream is stratified in a configuration which constitutes the desired multilayered coating to be contacted with the moving web. The end of the last slide is spaced apart from the moving web so that the multilayered stream exiting the last slide toward the web forms a bead or bridge between the last slide and the moving web. A pressure differential generally is effected across the bead by applying a vacuum to the bottom surface of the bead immediately adjacent the top surface of the web to stabilize the bead against excessive vibration and rupture. As the web contacts the bead, it entrains the multilayered liquid, thereby becoming coated. Typical examples of the apparatus described above are set forth in U.S. Pat. Nos. 2,761,419; 3,220,877; 3,928,678 and 3,928,679.
The coating apparatus described is useful, for example, to form webs coated with superimposed layers of aqueous photographic compositions including light sensitive materials, chemical sensitizers, antifoggants, developing agents and the like. These compositions are mixed with synthetic or naturally occurring colloids such as gelatin, polyvinyl compounds, or the like, which form non-flowing set layers containing the photographic compositions when the colloid is dried on the web. Unfortunately, oftentimes, chemical or physical reactions occur between adjacent layers to form an intermediate thin third layer as the liquids cascade down the slide apparatus. As the multilayer composition is entrained, the third layer, since it is generally non-uniform, may cause undesirable discontinuities in the adjacent layers. In addition, when there is a wide difference in viscosity between adjacent layers, a hydrodynamic standing wave can form since the low viscosity layers accelerate faster than the higher viscosity layers. Such a standing wave causes non-uniform coatings on the moving web which results in unacceptable coated products, particularly photographic film products.
U.S. Pat. No. 2,932,855 discloses a method for forming a coated sheet by extruding a high viscosity, self-supporting, liquid film composition through a slot and into a bead or beads of low viscosity liquid which are extruded through slots adjacent the slot for the high viscosity liquid. The low viscosity liquids remain in contact with the high viscosity film by surface tension forces. The film coated with low viscosity liquid is entrained onto the surface of a moving drum or onto a web on the moving drum wherein it is dried. The disclosed method is undesirably limited since it requires the use of a self-supporting high viscosity film which cannot be used in a number of applications such as in photographic film. U.S. Pat. No. 3,526,528 discloses a coating apparatus for applying a plurality of liquid layers on a moving web. The liquid layers are extruded individually through slots located on a common feeding head and are coated directly and sequentially onto the moving web. The feeding head must be positioned quite close to the moving web, e.g., 0.2-0.5 mm since the edges of the slots function as doctor blades to regulate the thickness of each liquid layer. Because of this approach, the length of the bead formed between the applicator lip and the web is considerably long, causing particulates on the web to get trapped in the bead and cause streaking. When this occurs or whenever a slot becomes partially blocked with foreign solid matter, the coating procedure must be stopped to remove the feeding head away from the moving web to permit access to the blocked slot.
It would be desirable to provide a coating apparatus and method which assures forming uniform layers within a multilayer liquid composition to be coated on a moving web. In addition, it would be desirable to provide a coating apparatus and method which minimizes the time adjacent liquid layers are in contact in order to minimize reaction between adjacent layers.